Genetic variations occur in susceptibility to infection in Biomphalaria glabrata and infectivity in Schistosoma mansoni. Studies indicate at least 12 genetically different susceptibility types in B. glabrata. Thirteen strains or substrains of S. mansoni apparently differing genetically in snail infectivity are being compared. Ten of these are of Puerto Rican origin. Two different mechanisms of insusceptibility to S. mansoni in various genetic stocks of B. glabrata are being compared: active resistance with amebocytic encapsulation and destruction, unsuitability for parasite development in the absence of observed host tissue reaction. Genetic tendency in B. glabrata for amebocytic accumulations, and relationship between this and insusceptibility to infection with S. mansoni are being studied. B. glabrata, when infected with x-irradiated miracidia of certain species of trematodes, acquires specific resistance to a subsequent challenge with nonirradiated miracidia of the same species. In addition, preinfection with nonschistosome miracidia interfers with the active resistance of the snails to S. mansoni, rendering them temporarily susceptible. These phenomena are being studied with the trematodes Echinostoma paraensei and Ribieroa marini. The ability of three stocks of B. glabrata to develop resistance to the 4 most commonly used molluscicides is being compared. Successive generations of survivors of molluscicide exposures are being reared and tested for molluscicide resistance.